A259650 Largest prime factor of the n-th pentagonal number (A000326).

5, 3, 11, 7, 17, 7, 23, 13, 29, 11, 7, 19, 41, 11, 47, 17, 53, 19, 59, 31, 13, 23, 71, 37, 13, 5, 83, 43, 89, 31, 19, 11, 101, 13, 107, 37, 113, 29, 17, 61, 7, 43, 131, 67, 137, 47, 13, 73, 149, 19, 31, 79, 23, 41, 167, 19, 173, 59, 179, 61, 37, 47, 191, 97

Offset

2,1

Links

Colin Barker, Table of n, a(n) for n = 2..1000

Formula

a(n) = A006530(A000326(n)).

Example

a(3) = 3 because A000326(3) = 12 = 2^2 * 3.

Prog

(PARI)
pg(m, n) = (n^2*(m-2)-n*(m-4))/2 \\ n-th m-gonal number
lpf(m) = vecmax(factorint(m)[, 1]) \\ Largest prime factor
a(n) = lpf(pg(5, n))

Crossrefs

Cf. A000326, A006530, A259649, A259651, A259652.

Sequence in context: A258333 A137613 A335302 * A165670 A221869 A237116

Adjacent sequences: A259647 A259648 A259649 * A259651 A259652 A259653

Keyword

nonn

Author

Colin Barker, Jul 02 2015

Status

approved